Method and apparatus for drying continuous sheets

ABSTRACT

Methods of and apparatus for use in drying continuous sheets are disclosed with particular reference to paper making with each sheet being conveyed through a dryer with both surfaces subjected to such elevated temperatures that unequal transverse drying may result. In both method and apparatus, the sheet is transversely monitored to determine whether in a preselected transverse zone a margin is drying so much faster than a portion intermediate its margins that various defects will be present in the dried sheet. If such unequal drying is detected, the drying temperature at the infeed side of the monitored zone is modified until the moisture content of the sheet is approximately uniform throughout the width of the sheet in that zone to prevent the development of defects and to ensure that both production and the quality of the sheet are at a maximum value.

United States Patent Sawyer [75] Inventor: Willard C. Sawyer, Oxford, Maine [73] Assignee: Gorham International Inc., Gotham,

Maine [22] Filed: Apr. 9, 1973 [2]] Appl. No.: 348,934

[52] U.S. Cl 34/41, 31/48, 34/113, 34/112 [51] Int. Cl. F261) 13/08 [58] Field of Search 34/41, 48, 113, 152

[56] References Cited UNITED STATES PATENTS 2,484,594 8/1949 Spangenberg 34/48 2,611,974 9/1952 Stratveit et a1. 34/48 3,040,807 6/1962 Chope 34/48 3,119,560 l/1964 Swaney 34/48 3,518,775 7/1970 Bartles et al r r 34/48 [451 Feb. 11, 1975 Primary ExaminerCharles J. Myhre Assistant Examiner-Paul Devinsky ABSTRACT Methods of and apparatus for use in drying continuous sheets are disclosed with particular reference to paper making with each sheet being conveyed through a dryer with both surfaces subjected to such elevated temperatures that unequal transverse drying may result. In both method and apparatus, the sheet is transversely monitored to determine whether in a preselected transverse zone a margin is drying so much faster than a portion intermediate its margins that various defects will be present in the dried sheet. If such unequal drying is detected, the drying temperature at the infeed side of the monitored zone is modified until the moisture content of the sheet is approximately uniform throughout the width of the sheet in that zone to prevent the development of defects and to ensure that both production and the quality of the sheet are'at a maximum value.

15 Claims, 9 Drawing Figures P/JENTED I 3.864.842

SHEET 1 OF 2 FIG 2 I I |70F IL 155 F STAGE1 I STAGE 2 I STAGE 3 FRONT I BACK MIDDLE F/Gj FIG. 4

STAGE1 STAGE 2 STAGE 3 FRONT BACK 220F I 220F MID%)LE FIG 5 220F FIG. 6 ZOOOF T STAGE1 STAGE2 STAGE 3 FRONT BACK MIDPLE F/Gf 7 METHOD AND APPARATUS FOR DRYING CONTINUOUS SHEETS BACKGROUND OF THE INVENTION In paper production, there is always a demand for improved quality for there are certain desirable properties that it is desirable to impart to the highest possible degree to the paper during its production and there are a number of undesirable properties developed in the sheet during its production, the elimination or reduction of which would be advantageous.

The former include uniformity of moisture content, smoothness, uniformity of caliper, an increase in the elongation at rupture in the machine direction and a decrease in this property in a cross machine direction, a decrease in Youngs Modulus of elasticity in the machine direction and an increase in that property in the cross machine direction, a decrease in the tensile strength at rupture in the machine direction and an increase in this property in the cross machine direction, an increase in Gurley density, increased Mullen test, and increased area under the stress-strain curve.

Defects found in paper that are attributable to unequal transverse drying of a sheet include grainy edges, wet streaks, curl, wrinkles, stretched paper, corrugations, ropes, diamonds, and tire tracks. While many paper makers in many different paper mills must have encountered one or more or all of these same defects in various aspects and forms, the names given to the defects are known to vary from mill to mill and even from paper maker to paper maker. For that reason, the terms used herein for several defects are herein defined.

Grainy edges is a phenomonen which must occur on almost any paper web made on any production paper machine. As compared only to the middle portion across that same paper web, a strip in the machine direction, near each of the deckle edges of the web, is tougher in surface than the middle, higher in caliper for the same base weight, and more curl reactive to changes in the relative humidity of the surrounding atmosphere. The phenomenon is usually most pronounced at the deckle edges and it usually decreases gradually and progressively from the deckle edges towards the middle unit at some stage it becomes unobservable. Under the best of circumstances the grainy edges may be observable for only a few inches in from the deckle edges but under the worst of circumstances the effect may be observable for as much as eighteen inches or more in from either deckle edge. The grainy edges are more difficult to calender, or to coat and calender, than the middle of that same web. The grainy edges may cause the wound roll of the paper web to be either larger or harder than the middle portion.

Wet streaks mean a variable condition across the web such as might have been created by a moisture variation, transversely of the sheet, existing at an earlier condition, whether or not the wet streak is observable at the end of the dryer. Defects due to wet streaks may appear as wrinkles.

Stretched paper is paper in which some portion has been stretched under tension beyond its elastic limit while the elastic limit has not been exceeded in adjacent portions due to one portion drying faster than the other. When the tension is relieved the first named portion of the paper is permanently distorted.

Corrugations", ropes, diamonds and tire tracks" are defects appearing in rolled sheets and may be and often are due to the sheet having been dried with an unequal transverse moisture content.

These defects have persisted and even increased as more modern high speed dryers have been constructed. Higher production rates require either more drums or higher temperatures. It is more'economical to increase temperatures than to lengthen dryers so the trend has been toward higher temperatures. Along with this trend has come a trend in design toward the grouping of drying cylinders into two or more sections which usually are operated at different fixed temperatures in a traditional temperature progression, one section relative to another. Traditionally the outfeed section or sections are higher in temperature than the section or sections toward the infeed. It is recognized that, as more liquid is removed, the remaining liquid requires higher temperatures to remove it from the sheet while still maintaining an economical length to the overall dryer portion of the machine whether it is a papermaking machine, a coater or whatever other drying process is involved. Up until the disclosure contained in this application, it has not been recognized that these higher temperatures must be controlled with regard to the specific sheet conditions subsequently described to avoid the defects previously described and to enhance both quantity and quality of the product produced.

Proposals have been made to cope with some of these defects, see, for example, the following US. Pat. Nos. 3,176,411 and 3,262,344.

THE PRESENT INVENTION While the invention is concerned with the drum drying of sheet materials in general, it is herein discussed with particular reference to paper production and its general objectives may be best appreciated in connection therewith.

The general objectives of the invention are twofold. The first is to improve the quality of paper by more fully developing the above referred to desirable properties and, at the same time, by the same means and procedures, minimizing or eliminating the defects such as those to which reference has been made. The second is to create the necessary but heretofore unavailable control of the drying process which makes possible increased production with as much freedom from the defects previously mentioned as is desired and with the enhancement of the desirable properties previously mentioned.

At this point, it will prove helpful to summarize the operation of dryers for a paper sheet or web as it is advanced from the paper making machine to the next operation. Such references as papermaking and Paperboard Making, Vol. III, page 408, Library of Congress Catalog Card No. 68-20994 and Drying of Paper and Paperboard, Lockwood Publishing Co., Inc., 1972, page 4, establish that the drying process takes place in three stages. The first of these is termed the warm-up stage, the second the constant drying rate" stage and the third, the falling rate stage. These stages are concerned only with sheet or web conditions.

A typical dryer has a substantial number of cylindrical heated, rotatable drums with one common type having its drums arranged in two tiers disposed either horizontally or vertically, with the sheet, as it comes from the associated paper making machine, trained about the drums so that first one surface and then the other is in engagement with a drum.

During the first stage condition, the sheet becomes heated by its contact with several drums until, due to the increased vapor pressure of the water, the water will leave the sheet, if air is available capable of picking up the vapor and carrying it away. When this occurs, the sheet is in its second stage condition and the water vapor carried away is free water that was carried by the sheet as distinct from bound water that is absorbed or entrapped by the fibers and during this second stage, the temperature of the sheet remains constant as long as the temperature of the drums remains constant.

During the third stage condition, the water is not so easily removed as the water present in the sheet is now bound water that does not readily travel to the surfaces of the sheet.

While the existence of three drying stages is recognized, where stage two drying conditions stop and stage three drying conditions start in any dryer is not obvious but if samples were taken from the sheet from end-toend of the dryer and each was tested as to its water content as by determining whether or not a sample gains weight when exposed to air of 100% relative humidity, the first such sample gaining weight would already be in its third stage. When the drums are heated to a sufficiently high temperature, say l80F and above, such a cumbersome procedure is not needed as the temperature of the sheet rises as the free water disappears. As a consequence, there is a fairly abrupt and substantial temperature rise in the sheet to indicate the junction of stage two and three drying conditions and the moisture content of the sheet at this junction is often referred to as the critical moisture content. As an additional consequence, there is a fairly abrupt and substantial rise in the drum surface temperature also as a result of the changed condition within the sheet. This drum surface temperature rise is at first most noticeable at the edge and is a further deterrent to obtaining optimum quality and production.

In accordance with the invention, its several objectives are attained by ensuring that the margins of the sheet are not in a stage three condition while the transversely aligned central part of the sheet is still in a stage two drying condition as under such conditions of unequal drying some or all of the previously mentioned defects will be objectionably apparent in the dried sheet. The typical more rapid drying of the edges and the resulting rise in temperature of the edges as opposed to the middle is symptomatic of the defects previously mentioned and can be detected at the precise stage of the drying process where the defects are introduced by observing when the edge first rises in temperature faster than the center. Control of the drying process such as to eliminate these defects and to produce the quality benefits previously mentioned has been achieved in accordance with the invention by reducing the temperature of sufficient drums on the infeed side of the monitored zone, thereby reducing the drying rate, such that the temperature difference between the edge and the middle of the sheet falls between 2F and 5F, the edge temperature being the higher. The greatest edge effect so monitored lies in the outer 8 to 18 inch region of the sheet wherever the highest temperature is found to exist. There is no added benefit to be achieved by reducing this temperature differece to less than 2F and no ill effects observed by allowing the temperature difference to rise to 5F. Therefore a very useful temperature range is established which allows control both upward and downward to keep this important but heretofore unrecognized phenomenon of the drying under control. Where less than maximum quality can be tolerated by the manufacturer, in the case of coarse papers, for example, the temperature differences may be permitted to rise above 5F but not beyond about 10F.

It is not completely understood why the edges dry faster but it is believed to be related to the greater degree of unrestrained cross machine shrinkage present at the edge as opposed to the middle and to the resulting difference in the rate of migration of solid water to the sheet surface at the edge. Without fully understanding the mechanism other than to recognize that it is a rate phenomenon, the applicant has determined that a reduced drying rate provided at this stage of the process provides the time necessary for the drying of the edges and the middle to equalize. The result of such equalization is to allow the whole width of the sheet to enter the third, bound water stage at the same time. Once this objective is achieved, the temperature of the sheet may then be elevated to whatever temperature is necessary to achieve the desired production without edge-to-middle differences again appearing. The net result is a higher overall drying rate because this invention makes it possible to increase the drying rate both before and after the monitored zone, but allows a lowered drying rate at the precise stage of drying where a high rate introduces defects. Furthermore, this invention provides the necessary means to monitor and control this stage of the drying process.

In accordance with the invention, the sheet is monitored to determine whether a transverse zone exists in which such unequal drying conditions exist. In that event, the drying temperature at the infeed side of the monitored zone is lowered until the moisture content of the sheet is approximately uniform. Desirably the change in the drying temperature is effected by lowering the temperature of as few of the drums as possible. While the monitoring of the sheet may be otherwise determined, it is preferred to use temperature responsive means that may be either fixed or transversely oscillated. Available devices are capable of detecting and responding to temperatures that differ in a range of i 1F.

Apparatus in accordance with the invention may include, in addition to the monitoring means and the means to vary the drum temperature on the infeed side of the monitored zone, a proportional re-set controller operated in response to the monitoring means and varying the control or controls of selected drums until the moisture content of the sheet in the monitored zone is uniform enough to ensure the production of paper of the desired quality.

Conversely, if no edge-middle effect is noted and if higher production is desired, the controller is operable to raise the temperature of at least one drum for each web surface at the infeed side to increase the drying rate until the edge-middle effect is noted and then to control the edge-middle temperature difference to within such limits as are required to assure the quality level desired. The highest quality is achieved when the edge-middle effect is manifested by a temperature difference between 2F and 5F, the edge temperature being the higher.

The methods and apparatus as thus broadly defined and subsequently more completely detailed are particularly useful when the speed of old dryers is increased or new dryers of higher capacity are being designed.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, a dryer in accordance with the invention is schematically shown as are other views giving lengthwise and transverse temperature plots with various web temperatures.

In the drawings:

FIG. 1 is a somewhat schematic view of a dryer;

FIG. 2 is a lengthwise web temperature plot showing the temperature progression with the internal temperature of all the drums 180F;

FIG. 3 is a transverse web temperature plot showing the temperature profile in a zone where the middle of the sheet has just reached third stage drying condition as have its margins;

FIG. 4 is a lengthwise web temperature plot showing the temperature progression with the internal temperature of all the drums 230F;

FIG. 5 is a transverse web temperature plot showing the web temperature profile in a zone where the middle of the sheet has just reached stage three drying condition which the margins have already reached;

FIG. 6 is a lengthwise web temperature plot showing the temperature progression with the internal drum temperature 230F but with the dryer operated in accordance with the invention;

FIG. 7 is a transverse web temperature plot showing the web temperature profile in approximately the same zone as the plot shown in FIG. 5;

FIG. 8 is a schematic side view of a dryer in accordance with the invention illustrating the means by which the sensed temperature difference is employed to control drum temperatures at the infeed side of the junction between stages two and three to produce the web temperature plot shown in FIG. 7; and

FIG. 9 is a fragmentary schematic plan view of the junction zone of the dryer illustrated by FIG. 8.

The dryer indicated generally at 10 in FIG. 1 is schematically shown as it is not intended to represent or be typical of any existing machine but rather to provide a basis, first, to discuss conditions at both ends thereof, and second, to illustrate certain of its own essential requirements.

As to the first, the dryer 10 receives the paper sheet or web 11 at its left hand or infeed end from the press section of a paper machine in a conventional manner and is subject to a maintained degree of imposed and material tension throughout the dryer as is conventional. The sheet 11 from the dryer passes through the calender 12 or other processing apparatus and when it leaves the dryer, it must have certain standards, by way of example, uniform and constant weight, moisture content, and finish.

The essential features of the dryers are that the drums l3 and 14 of the upper and lower tiers, respectively, be all of the same diameter, clean, and so driven together that all will have a surface speed properly coordinated with the sheet speed and that the sheet 11 be trained through the dryer 10 in engagement first with a drum of one tier and then in engagement with a drum of the other tier. The drums are steam heated with the heating system having a control by which the temperature of the drums can be raised or lowered. For reasons that will subsequently be apparent, dryers in accordance with the invention may have more than the number of drums than are needed to dry the sheet conventionally. Each drum is internally heated to a predetermined temperature as by steam delivered thereto, see FIG. 8, via branch lines 15 from a main l6 and each branch line 15 includes an adjustable valve 17 of a type enabling the drum temperature to be controlled accurately. While the temperature of each drum of each tier can be individually controlled, each valve 17 is shown as controlling the temperature of one drum of each tier. The steam source must be capable of heating each drum to a predetermined maximum temperature, say 350 F or higher, and each valve 17 must be adjustable to establish a drum temperature within a predetermined temperature range, say l40-350F or higher. The internal temperature of each drum must be uni form throughout the entire area of its web contacting surface.

In the operation of the dryer 10, there are, as previously noted, three stages of web drying conditions of which the warm-up stage involves whatever number of drums at the wet or infeed end are required to elevate the web temperature from its starting temperature to whatever the web temperature becomes in the constant rate stage of drying. The second or constant rate stage contains whatever number of drums are necessary to so treat the sheet that the cooling effect of the evaporation of water pick-up by the air is constant, indicating that in stage two the margins have not reached a stage three drying condition. When, however, the amount of water vapor available for removal drops at a margin of a transverse zone, the temperature of the sheet 11 rises, signalling the commencement of stage three drying condition.

In stage 3 drying condition, the water contained within the web does not travel to its surfaces as readily as it did in stage 2 because the moisture is now largely bound water in the fibers or entrapped thereby. As was earlier noted, both surface and entrapped moisture is more readily removed at the margin than it is a short distance inwardly thereof.

Turning now to a first assumed plot of a temperature progression lengthwise of a dryer centrally of the sheet and having an undetermined number of drums, each controlled to have a surface temperature of F and there being enough drums to dry the sheet 11 down to the desired moisture content, the temperature of the sheet remaining constant throughout stages 2 and 3 because the low temperature established a drying rate that is, at all times slow enough to enable the moisture entrapped within the sheet to migrate to its surfaces. The junction between stage 2 and 3 can be determined, although such is not necessary, by taking samples after each drum in succession until one is found that takes up weight in atmosphere controlled at 100% R.H., that sample being in stage 3 drying condition.

Such a dryer would result in paper having a uniform water content transversely. It would, of course, be highly impractical because it would require so many additional drums that it would be not only too costly but also too long for use in most locations.

It will now be assumed that the internal temperature of each drum is controlled to be at or above 180F so that the sheet has, when in its second stage condition, a temperature at or correspondingly above l55F-, see FIG. 2. Two factors change immediately. In the first place, it is obvious that fewer drums are now required to dry the same sheet to the same extent. In the second place, the conditions that separate stage 2 from stage 3 can now be detected by measurement of the web or drum surface temperature, preferably the former, as well as by sample testing. This is so because the drying rate has been accelerated and it will be noted that the web temperature progression is plotted lengthwise of the dryer and centrally of the web to determine where the middle portion of the sheet enters its stage three condition and the temperature of the transverse zone inclusive of that area is monitored to determine whether the margin or margins of the sheet are already then in a stage three drying condition. If so, the margins are drying too rapidly. For that reason, the temperature in a sufficient number of drums on the infeed side of that location is lowered by the adjustment of certain ones of the valves, the valves 17A, until the web temperatures throughout that transverse zone are approximately equal. Drying to the desired moisture content may require an increase in drum temperatures in the outfeed side of the monitored zone unless the dryer has a few more drums than would be needed to dry the sheet conventionally. If no edge-middle effect is noted in the monitored zone and it increased production is wanted, the temperature of at least some of the drums on the infeed side of that zone may be increased as by adjusting the valve 17B in control thereof with the continued monitoring of the selected transverse zone safeguarding the unequal drying that would result in the creation of defects and to ensure the desired quality level. It is preferred that the drum temperature modification be confined to as few drums as possible as suggested in FIG. 6 by the dip in the sheet temperature to l90F.

In FIG. 8, the dryer of FIG. 1 is shown as modified in accordance with the invention to enable its functioning to be controlled in response to conditions as they are encountered by the means for monitoring the selected transverse zone.

While the monitoring of that critical transverse zone may be done otherwise than by temperature comparison, that approach is preferred and for this purpose a heat sensor indicated at 18 in FIG. 8, a Williamson Model 1400 Industrial Radiometer, for example, may be used to operate a conventional proportional, re-set controller 19 to adjust the valves 17A by increments until the drum temperatures are corrected upwardly or downwardly to eliminate the sensed temperature difference. Such a controller may be API Model 227, API Instrument Co., Division of L.F.E. Corporation of Waltham, Massachusetts. Desirably, the testing of the sheet in the machine direction to determine whether and where a critical zone exists is effected with a similar but protable heat sensor 18A advanced along a path 20 to test the sheet 11 on the outfeed side of and close to each drum 13, 14, the testing zones indicated at T1 through T8, until the critical zone is located at T8 for the sensor 18.

While there may be several heat sensors spaced across the sheet to monitor the entire zone, it is preferred to use a transversely oscillating sensor adjacent one or the other or both to detect transverse moisture content variations that will result, unless corrected, in the presence of defects in the dried sheet with a consequential loss of quality.

While the text and diagram of this application have called for individual control of each cylinder so that maximum flexibility was provided for explanation it is recognized that it would be possible and economically desirable to group certain dryers on common control.

I claim:

.l. The method of drying a wet sheet that comprises the steps of conveying the sheet along a path with both surfaces in contact with rotatable, steam-heated drums establishing drum temperatures such that at the outfeed end, the sheet has a desired moisture content and during said drying has a zone where its moisture content becomes critical between an infeed zone where the drying rate is constant and an outfeed zone where the drying rate is falling, monitoring the sheet to determine whether in that zone the moisture content of at least one margin of the sheet is critical in that its moisture content is bound water and the intermediate portion still contains free water, and then, if such a difference in moisture content is detected, reducing the temperature of drums in the outfeed end of the zone of constant rate drying to an extent such that the moisture content of the sheet in the monitored zone becomes approximately uniform thereby to avoid defects that would result if the margin became overdried before the free water was removed.

2. The method of claim 1 in which the sheet is a continuous length of a paper sheet passing from a paper making machine to another processing apparatus.

3. The method of claim 1 in which the heat modifying step also includes the increase in the temperature of drums in a predetermined lengthwise zone but on the infeed side thereof remote from said monitored zone.

4. The method of claim 1 in which the monitoring involves the comparison of the temperature of at least one of the margins with a portion of the sheet that is representative of the middle thereof.

5. The method of claim 4 and the step of modifying the heat is such as to result in the difference in the temperature of the margin and said portion being maintained in the approximate range of from 2 to 5F, the temperature of the margins being the higher.

6. The method of claim 1 in which the monitoring consists of the steps of first taking the temperature of the sheet lengthwise thereof to determine where a zone exists in which there is an increase in its temperature without any corresponding increase in the applied heat, and then monitoring the zone transversely.

7. The method of claim 6 in which the transverse monitoring is effected by comparing the temperature of at least one margin of the sheet with a portion spaced transversely thereof that has a potentially slower drying rate than the margin thereof to determine whether the temperature of the former is higher than the temperature of the latter.

8. A dryer for a wet sheet including a plurality of rotatable drums arranged in two series so that the sheet may be in trained engagement with the drums with first one sheet surface and then the other sheet surface in contact with the drums of the appropriate series and with the sheet continuously advanced thruogh the dryer by said drums, means to heat said drums to a temperature such that at the outfeed end of the dryer, the sheet has a desired moisture content and that, in a zone transversely of the sheet between the zone where the drying rate is constant and the zone where the drying rate is falling, the moisture content becomes critical, means operable to monitor the sheet in said intermediate zone to determine whether the moisture content'of at least one margin of the sheet has become critical in that its moisture content is bound water and an intermediate portion of the sheet still contains free water, and means operable if such a difference in moisture content is detected, to reduce the temperatureof the applied heat in a transverse zone close to and on the infeed side of the monitored zone to an extent such that the moisture content of the sheet throughout the monitored zone becomes approximately equal thereby to prevent the development of defects in the sheet that would result if the margin became over-dried before the free water was removed from the intermediate portion of the sheet.

9. The dryer of claim 8 in which the heat modifying means is operable to lower from a predetermined value the temperature of at least one drum for each sheet surface adjacent said zone.

10. The dryer of claim 9 in which the heat modifying means includes means operable to increase from a predetermined value the temperature of at least one drum for each sheet surface remote from said zone but on the infeed side thereof.

11. The dryer of claim 8 in which said limits are in the approximate range of from 2 to 5F, the marginal temperature being the higher.

12. The dryer of claim 8 in which the monitoring means is temperature responsive.

13. The dryer of claim 8 and a proportional reset controller operatively connected to the temperature modifying means and operable by said monitoring means.

14. The dryer of claim 8 in which the monitoring means is an oscillating temperature sensor.

15. The dryer of claim 12 in which the means operable in response to the monitoring means is operable to maintain the temperature differential in the monitored zone in the approximate range of from 2 to 5F, the

marginal temperature being the higher. 

1. The method of drying a wet sheet that comprises the steps of conveying the sheet along a path with both surfaces in contact with rotatable, steam-heated drums establishing drum temperatures such that at the outfeed end, the sheet has a desired moisture content and during said drying has a zone where its moisture content becomes critical between an infeed zone where the drying rate is constant and an outfeed zone where the drying rate is falling, monitoring the sheet to determine whether in that zone the moisture content of at least one margin of the sheet is critical in that its moisture content is bound water and the intermediate portion still contains free water, and then, if such a difference in moisture content is detected, reducing the temperature of drums in the outfeed end of the zone of constant rate drying to an extent such that the moisture content of the sheet in the monitored zone becomes approximately uniform thereby to avoid defects that would result if the margin became overdried before the free water was removed.
 2. The method of claim 1 in which the sheet is a continuous length of a paper sheet passing from a paper making machine to another processing apparatus.
 3. The method of claim 1 in which the heat modifying step also includes the increase in the temperature of drums in a predetermined lengthwise zone but on the infeed side thereof remote from said monitored zone.
 4. The method of claim 1 in which the monitoring involves the comparison of the temperature of at least one of the margins with a portion of the sheet that is representative of the middle thereof.
 5. The method of claim 4 and the step of modifying the heat is such as to result in the difference in the temperature of the margin and said portion being maintained in the approximate range of from 2* to 5*F, the temperature of the margins being the higher.
 6. The method of claim 1 in which the monitoring consists of the steps of first taking the temperature of the sheet lengthwise thereof to determine wheRe a zone exists in which there is an increase in its temperature without any corresponding increase in the applied heat, and then monitoring the zone transversely.
 7. The method of claim 6 in which the transverse monitoring is effected by comparing the temperature of at least one margin of the sheet with a portion spaced transversely thereof that has a potentially slower drying rate than the margin thereof to determine whether the temperature of the former is higher than the temperature of the latter.
 8. A dryer for a wet sheet including a plurality of rotatable drums arranged in two series so that the sheet may be in trained engagement with the drums with first one sheet surface and then the other sheet surface in contact with the drums of the appropriate series and with the sheet continuously advanced thruogh the dryer by said drums, means to heat said drums to a temperature such that at the outfeed end of the dryer, the sheet has a desired moisture content and that, in a zone transversely of the sheet between the zone where the drying rate is constant and the zone where the drying rate is falling, the moisture content becomes critical, means operable to monitor the sheet in said intermediate zone to determine whether the moisture content of at least one margin of the sheet has become critical in that its moisture content is bound water and an intermediate portion of the sheet still contains free water, and means operable if such a difference in moisture content is detected, to reduce the temperature of the applied heat in a transverse zone close to and on the infeed side of the monitored zone to an extent such that the moisture content of the sheet throughout the monitored zone becomes approximately equal thereby to prevent the development of defects in the sheet that would result if the margin became over-dried before the free water was removed from the intermediate portion of the sheet.
 9. The dryer of claim 8 in which the heat modifying means is operable to lower from a predetermined value the temperature of at least one drum for each sheet surface adjacent said zone.
 10. The dryer of claim 9 in which the heat modifying means includes means operable to increase from a predetermined value the temperature of at least one drum for each sheet surface remote from said zone but on the infeed side thereof.
 11. The dryer of claim 8 in which said limits are in the approximate range of from 2* to 5*F, the marginal temperature being the higher.
 12. The dryer of claim 8 in which the monitoring means is temperature responsive.
 13. The dryer of claim 8 and a proportional reset controller operatively connected to the temperature modifying means and operable by said monitoring means.
 14. The dryer of claim 8 in which the monitoring means is an oscillating temperature sensor.
 15. The dryer of claim 12 in which the means operable in response to the monitoring means is operable to maintain the temperature differential in the monitored zone in the approximate range of from 2* to 5*F, the marginal temperature being the higher. 